This invention relates to a device for measuring a printing area (or a printing image area) by using a printing plate for an offset printing machine, and more particularly to a printing image area measuring device which can detect the surface of a printing plate with high accuracy even if the surface is uneven.
In addition to a method of this type, many printing image area measuring methods are known in the art. For instance, there are methods of using proofs, actual prints, reflection copies, transparent copies and so on. Furthermore, known in the art are a method in which the printing image area measurement value is utilized to check the density of prints during the operation of the printing machine to feed back the density thus checked to the printing machine, and a method in which the printing image area measurement value is used to preset an amount of ink to be supplied before a printing operation is carried out.
In general, the printing image of a printing plate is obtained by subjecting a plurality of copies arranged in place to printing. Therefore, the methods other than those in which printing plates or prints are utilized for measuring printing image areas are disadvantageous in that it is necessary that, after individual printing image areas have been measured, the measurement data are summed with the layout on the printing plate taken into account. On the other hand, in the method of utilizing printing plates or prints for obtaining measurement data, the measurement data can be immediately used. In the case of obtaining measurement data from a print, a feed-back control system correcting variations due to external disturbance during a printing operation is established because the measurement is effected after the printing operation has been started. In the case of obtaining measurement data from a printing plate, it is intended to provide satisfactory prints even at the start of a printing operation by presetting the degrees of opening of the ink adjusting keys before the printing operation is started.
A variety of devices for measuring printing image areas by using a printing plate are known in the art. In a first example of the conventional device, a printing plate wrapped around a cylinder is rotated at high speed, and each of the average value of the afterimages of divisional bands is measured (cf. Japanese Patent Application Publication No. 42205/1972). In a second example, an offset printing plate is scanned to obtain the number of pulses corresponding to the printing area, thereby to adjust the amount of ink to be supplied (cf. Japanese Patent Application Laid-Open No. 53804/1973). In a third example, the printing area of a printing plate is detected and integrated for every ink adjusting key, the intensity of reflection light from the nonprinting area is detected by using an auxiliary printing plate, and signals representative of the printing area only are processed by using the detection signals of the printing plate to provide a value corresponding to the printing area, thereby to control the amount of ink to be supplied (cf. Japanese Patent Application Laid-Open No. 67714/1974). In a fourth example, a printing plate is scanned longitudinally to measure the printing area occupation percentage in a widthwise direction, whereby the amount of ink to be supplied is controlled in the ink fountain roller section (cf. Japanese Patent Application Laid-Open No. 2505/1976). In a fifth example, a photoelectric detector disposed in place laterally of a printing surface or the surface of a printing plate is moved longitudinally in a scanning manner to obtain the total amount of ink, thereby to control the amount of ink to be supplied.
However, these conventional devices are not practical because printing image areas cannot be detected with high accuracy. This will be described in more detail.
A printing plate is subjected to exposure through a transparent film copy placed in close contact therewith, and is then processed in accordance with processing steps shown in FIG. 1. In developing a positive type printing plate used usually (Step S1), a portion of the photosensitive layer which is exposed to light (i.e. a non-printing area) is removed so that no ink is applied thereto. However, after the development an unnecessary photosensitive layer may be partially left on the printing plate, and it is dissolved by applying an erasing liquid to it only (Step S3). After the printing plate has been dried (Step S4), a counter-etching liquid is applied to the entire surface of the printing plate (Step S5). Thereafter, the printing plate is subjected to buff drying to dry the counter-etching liquid on the surface of the printing plate (Step S7). The counter-etching and buff drying processes are preliminary processes effected before a burning (high temperature treatment) process is carried out. The printing plate is subjected to burning (Step S8), as a result which the endurance of the printing plate is increased two or three times. Finally, the printing plate is subjected to so-called "gumming" in order to protect the surface of the nonprinting area and to increase the hydrophilic characteristic of the printing plate (Step S10). Thus, all the steps of processing the printing plate have been accomplished.
In the above-described steps of process, the step of burning is considerably effective in improving the endurance of the printing plate. However, since a printing plate is heated at high temperatures ranging from 250.degree. C. to 300.degree. C. in the step of burning, an ordinary printing plate having a hard aluminum plate as its base material is thermally deformed. This deformation remains after the printing plate has been cooled, and therefore the flatness of the printing plate is reduced. Accordingly, if a printing image area is obtained from such a printing plate by detecting the intensity of light reflected thereby, then the intensity of light reflected is varied depending on the printing image area or with the extent of unevenness of the printing plate or with the deviation in position of the printing plate, as result of which it is impossible to correctly detect the printing image area and accordingly it is impossible to correctly determine the amount of ink to be supplied.
In the case where a number of printing machines different in kind are available, usually a variety of printing plates are processed in the step of printing with printing plates. Therefore, it is desirable that a meter for measuring a printing image area by using a printing plate is used integrally with a printing line at a printing site instead of a printing machine at the printing site; that is, it is desirable to employ a method in which data for controlling the amounts of ink of a number of printing machines are measured with one measuring device.